Enhance nbtree index tuple deletion.

Teach nbtree and heapam to cooperate in order to eagerly remove
duplicate tuples representing dead MVCC versions.  This is "bottom-up
deletion".  Each bottom-up deletion pass is triggered lazily in response
to a flood of versions on an nbtree leaf page.  This usually involves a
"logically unchanged index" hint (these are produced by the executor
mechanism added by commit 9dc718bd).

The immediate goal of bottom-up index deletion is to avoid "unnecessary"
page splits caused entirely by version duplicates.  It naturally has an
even more useful effect, though: it acts as a backstop against
accumulating an excessive number of index tuple versions for any given
_logical row_.  Bottom-up index deletion complements what we might now
call "top-down index deletion": index vacuuming performed by VACUUM.
Bottom-up index deletion responds to the immediate local needs of
queries, while leaving it up to autovacuum to perform infrequent clean
sweeps of the index.  The overall effect is to avoid certain
pathological performance issues related to "version churn" from UPDATEs.

The previous tableam interface used by index AMs to perform tuple
deletion (the table_compute_xid_horizon_for_tuples() function) has been
replaced with a new interface that supports certain new requirements.
Many (perhaps all) of the capabilities added to nbtree by this commit
could also be extended to other index AMs.  That is left as work for a
later commit.

Extend deletion of LP_DEAD-marked index tuples in nbtree by adding logic
to consider extra index tuples (that are not LP_DEAD-marked) for
deletion in passing.  This increases the number of index tuples deleted
significantly in many cases.  The LP_DEAD deletion process (which is now
called "simple deletion" to clearly distinguish it from bottom-up
deletion) won't usually need to visit any extra table blocks to check
these extra tuples.  We have to visit the same table blocks anyway to
generate a latestRemovedXid value (at least in the common case where the
index deletion operation's WAL record needs such a value).

Testing has shown that the "extra tuples" simple deletion enhancement
increases the number of index tuples deleted with almost any workload
that has LP_DEAD bits set in leaf pages.  That is, it almost never fails
to delete at least a few extra index tuples.  It helps most of all in
cases that happen to naturally have a lot of delete-safe tuples.  It's
not uncommon for an individual deletion operation to end up deleting an
order of magnitude more index tuples compared to the old naive approach
(e.g., custom instrumentation of the patch shows that this happens
fairly often when the regression tests are run).

Add a further enhancement that augments simple deletion and bottom-up
deletion in indexes that make use of deduplication: Teach nbtree's
_bt_delitems_delete() function to support granular TID deletion in
posting list tuples.  It is now possible to delete individual TIDs from
posting list tuples provided the TIDs have a tableam block number of a
table block that gets visited as part of the deletion process (visiting
the table block can be triggered directly or indirectly).  Setting the
LP_DEAD bit of a posting list tuple is still an all-or-nothing thing,
but that matters much less now that deletion only needs to start out
with the right _general_ idea about which index tuples are deletable.

Bump XLOG_PAGE_MAGIC because xl_btree_delete changed.

No bump in BTREE_VERSION, since there are no changes to the on-disk
representation of nbtree indexes.  Indexes built on PostgreSQL 12 or
PostgreSQL 13 will automatically benefit from bottom-up index deletion
(i.e. no reindexing required) following a pg_upgrade.  The enhancement
to simple deletion is available with all B-Tree indexes following a
pg_upgrade, no matter what PostgreSQL version the user upgrades from.

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Heikki Linnakangas <hlinnaka@iki.fi>
Reviewed-By: Victor Yegorov <vyegorov@gmail.com>
Discussion: https://postgr.es/m/CAH2-Wzm+maE3apHB8NOtmM=p-DO65j2V5GzAWCOEEuy3JZgb2g@mail.gmail.com

doc/src/sgml/ref/create_index.sgml

@@ -386,17 +386,39 @@ CREATE [ UNIQUE ] INDEX [ CONCURRENTLY ] [ [ IF NOT EXISTS ] <replaceable class=
      <para>
       The fillfactor for an index is a percentage that determines how full
       the index method will try to pack index pages.  For B-trees, leaf pages
-      are filled to this percentage during initial index build, and also
+      are filled to this percentage during initial index builds, and also
       when extending the index at the right (adding new largest key values).
       If pages
       subsequently become completely full, they will be split, leading to
-      gradual degradation in the index's efficiency.  B-trees use a default
+      fragmentation of the on-disk index structure.  B-trees use a default
       fillfactor of 90, but any integer value from 10 to 100 can be selected.
-      If the table is static then fillfactor 100 is best to minimize the
-      index's physical size, but for heavily updated tables a smaller
-      fillfactor is better to minimize the need for page splits.  The
-      other index methods use fillfactor in different but roughly analogous
-      ways; the default fillfactor varies between methods.
+     </para>
+     <para>
+      B-tree indexes on tables where many inserts and/or updates are
+      anticipated can benefit from lower fillfactor settings at
+      <command>CREATE INDEX</command> time (following bulk loading into the
+      table).  Values in the range of 50 - 90 can usefully <quote>smooth
+       out</quote> the <emphasis>rate</emphasis> of page splits during the
+      early life of the B-tree index (lowering fillfactor like this may even
+      lower the absolute number of page splits, though this effect is highly
+      workload dependent).  The B-tree bottom-up index deletion technique
+      described in <xref linkend="btree-deletion"/> is dependent on having
+      some <quote>extra</quote> space on pages to store <quote>extra</quote>
+      tuple versions, and so can be affected by fillfactor (though the effect
+      is usually not significant).
+     </para>
+     <para>
+      In other specific cases it might be useful to increase fillfactor to
+      100 at <command>CREATE INDEX</command> time as a way of maximizing
+      space utilization.  You should only consider this when you are
+      completely sure that the table is static (i.e. that it will never be
+      affected by either inserts or updates).  A fillfactor setting of 100
+      otherwise risks <emphasis>harming</emphasis> performance: even a few
+      updates or inserts will cause a sudden flood of page splits.
+     </para>
+     <para>
+      The other index methods use fillfactor in different but roughly
+      analogous ways; the default fillfactor varies between methods.
      </para>
     </listitem>
    </varlistentry>